1. Field of the Invention
The present invention relates to a photoreceptor for electrophotography (simply referred to as “photoreceptor,” hereinafter) for use in electrophotographic printers, copiers, facsimile machines and the like, a method for manufacturing the photoreceptor for electrophotography, and an electrophotographic apparatus. More particularly, the present invention relates to a photoreceptor for electrophotography that exhibits excellent wear resistance, photoresponsivity, and gas resistance by having a combination of a resin binder, a charge transport material, and an additive having specific structures, a method for manufacturing such photoreceptor for electrophotography, and an electrophotographic apparatus.
2. Description of the Related Art
A photoreceptor for electrophotography has a fundamental structure in which a photosensitive layer with a photoconductive function is placed on a conductive substrate. In recent years, research and development has been actively carried out on organic photoreceptors for electrophotography that use organic compounds as functional components responsible for the generation and transportation of charges, in view of advantages such as the diversity of materials, high productivity and safety, and application of such organic photoreceptors to copying machines, printers and the like is underway.
In general, photoreceptors are required to have a function of retaining surface charges in a dark place, a function of receiving light and generating charges, and a function of transporting the generated charges. Such photoreceptors are classified into so-called single layer type photoreceptors which have a single layer of photosensitive layer with a combination of these functions, and so-called laminated type photoreceptors (function-separated type) which include functionally separated layers such as a charge generation layer that is mainly in charge of generating charges at the time of light reception, a charge transport layer that is in charge of retaining surface charges in a dark place and transporting the charges generated in the charge generation layer at the time of light reception, and a photosensitive layer.
The photosensitive layer is generally formed by applying, on a conductive substrate, coating liquid prepared by dissolving or dispersing a charge generating material, a charge transport material and a resin binder in an organic solvent. In these organic photoreceptors for electrophotography, particularly in the layer that serves as the outermost surface, polycarbonate is often used as the resin binder. This is because polycarbonate is strongly resistant to the friction that occurs between the layer and paper or a blade for toner removal, has excellent flexibility, and has good permeability of exposure light. Among others, bisphenol Z type polycarbonate is widely used as the resin binder. Technologies of using such a polycarbonate as a resin binder are described in, for example, Japanese Patent Application Publication No. S61-62040 (Patent Document 1) and the like. In addition to the above, various studies on polycarbonate structures have been implemented to date for the purpose of enhancing wear resistance, but no satisfactory structures have been developed yet.
Meanwhile, with the recent increase in the number of prints resulting from the networking in offices and the rapid development of light printers due to electrophotography, higher durability, higher sensitivity, and faster responsiveness have been required for the electrophotographic printers. Moreover, the electrophotographic printers are strongly demanded to have less fluctuations in image characteristics which are usually caused by the ozone, NOx or other gas in the printers and by changes in the usage environment (room temperature and humidity).
In addition, with recent color printers being further advanced technically and becoming more common, the increase in print speed, the reduction in printer size, and the reduction in the number of printer components have been implemented. Along with this, color printers are required to be compatible with a wider range of usage. In color printers, stronger transfer currents have had to be used because a process of transferring overlapped toners and a transfer belt have been employed; thus, when performing printing on sheets of various sizes, the difference in transfer burden occurs between the paper sizes and between the sheets, resulting in an increase in the difference in image density. In other words, in case of printing on a large number of small sheets, the photoreceptor portion through which the sheets do not pass (non-passage portion) is constantly under a direct impact of transfer, compared to the photoreceptor part through which the sheets pass (passage portion), increasing the transfer burden. Due to this difference in transfer burden between the passage portion and the non-passage portion, a potential difference is generated in the developer when printing is performed subsequently on large sheets, creating a density difference. This tendency becomes more significant as the transfer current increases. Under such circumstances, compared to monochrome printers, the color printers in particular have less fluctuations in image characteristics and electrical properties which are caused due to repeated use or changes in usage environment (room temperature and humidity), and the demand for the photoreceptor with excellent transfer recoverability has been stronger. Therefore, the conventional technologies, unfortunately, cannot sufficiently fulfill such demand.
In order to improve wear resistance of a negatively-charged laminated-type photoreceptor, it is necessary to increase the ratio of a resin binder contained in a charge transport layer configuring the outermost layer. In so doing, the charge mobility of a charge transport material drops as a result of relatively reducing the ratio of the charge transport material. The charge mobility of the charge transport material needs to be improved, in order to solve this problem. In addition, with the compatibility between the resin binder and the charge transport material in mind, not only is it necessary to select a combination of a resin binder and a charge transport material, but also the ratios thereof need to be adjusted.
Ozone has widely been known as the gas generated in an electrophotographic apparatus. Ozone is generated by a charge that performs corona discharge or a roller charger. When the photoreceptor is exposed to the ozone remaining or accumulating in the apparatus, the organic substances configuring the photoreceptor are oxidized, destroying the original structure and significantly deteriorating the properties of the photoreceptor. The ozone also oxidizes the nitrogen in the air, producing NOx, which is considered to degenerate the organic substances configuring the photoreceptor.
Such degradation of the properties of the photoreceptor caused by the gas involves invasion of the outermost layer of the photoreceptor and an adverse effect caused by the gas flowing into the photosensitive layer. The outermost layer of the photoreceptor could be scraped off by friction between the outermost layer and the various members described above, depending on the degree. However, the harmful gas flowing into the photosensitive layer can destroy the structures of the organic substances of the photosensitive layer. Thus, it is important to consider a way to prevent the harmful gas from flowing into the photosensitive layer. Especially in a tandem system color electrophotographic apparatus using a plurality of photoreceptors, when gas affects the photoreceptors of the apparatus differently depending on where the photoreceptors are installed, fluctuations in color tones occur, interfering with proper generation of images. Such degradation of the properties of the photoreceptor in a tandem system color electrophotographic apparatus, therefore, is a particularly critical issue.
In some cases, the surface of the photoreceptor is contaminated by ozone, nitrogen oxides and the like that are generated at the time of charging the photoreceptor. When this happens, there are problems such as image bleeding due to the contaminants themselves, a decrease in lubricity of the surface of the photoreceptor caused by adhering materials, easy adhesion of paper dust and toner, squealing and peeling of the blade, and the susceptibility of the surface to scratches.
Various improvement technologies for the outermost layers of photoreceptors have been proposed for the purpose of solving these problems.
Various polycarbonate resin structures have been proposed for the purpose of improving the durability of a photoreceptor surface. Japanese Patent Application Publication Nos. 2004-354759 and H4-179961 (Patent Documents 2 and 3), for example, each propose polycarbonate resin having a specific structure, but do not take into enough consideration the compatibility between various charge transport materials and additives, as well as the solubility of the resin. Japanese Patent Application Publication No. 2004-85644 (Patent Document 4) also proposes polycarbonate resin having a specific structure. However, resin with a bulky structure has a lot of spaces between polymers, and, for example, discharged substances upon charging, contact members, or foreign matters may easily penetrate into the photosensitive layer, hence it difficult to obtain sufficient durability. In addition, Japanese Patent Application Publication No. H3-273256 (Patent Document 5) proposes polycarbonate having a special structure that is configured to improve printing durability and coatability, but does not provide enough description of a charge transport material or an additive to be combined, bringing about a problem in which stable electric properties cannot be maintained when the apparatus is used for a long time.
Various charge transport materials with high responsivity and high carrier mobility have been proposed as well. For example, Japanese Patent Application Publication No. S59-216853 (Patent Document 6) proposes a stilbene derivative, and Japanese Patent Application Publication No. 2012-27139 (Patent Document 7) a tris(4-styrylphenyl) amine derivative and the like. These patent documents, however, do not take into enough consideration resin binders or additives to be combined with the charge transport materials, and it has not been possible yet to comply with a change in operating environments, maintain the electrical properties of the photoreceptors used for a long time, improve wear resistance, and mainten contamination resistance.
For the purpose of improving gas resistance, various additives have been proposed, such as hindered phenol compounds, phosphorus-based compounds, sulfer-based compounds, amine-based compounds, hindered amine compounds. Unfortunately, the reality is that these technologies cannot obtain a photoreceptor having sufficient gas resistance or cannot accomplish satisfactory results regarding the electrical properties such as responsivity, image memories, and potential stability at the time of printing, depending on the combination of resin and the charge transport material, even if satisfactory characteristics are exercised for the gas resistance. On the other hand, the applicants propose diester compounds in WO2011/108064 and Japanese Patent Application Publication No. 2007-279446 (Patent Documents 8 and 9) and have been studying a more appropriate combination of a resin binder and a highly mobile charge transport material.
Various types of technologies for improving the surface layers of the photoreceptors have been proposed, as described above. Unfortunately, the technologies described in these patent documents are not satisfactory in all aspects of electrical properties such as photoresponsivity, wear resistance, solvent crack resistance and the like.
An object of the present invention, therefore, is to provide a photoreceptor for electrophotography that exhibits high photoresponsivity, stable electrical properties, and high durability even when used repeatedly. More specifically, an object of the present invention is to provide a photoreceptor for electrophotography of excellent wear resistance, responsivity, and gas resistance by having a combination of a resin binder, a charge transport material, and an additive having specific structures, a method for manufacturing such a photoreceptor for electrophotography, and an electrophotographic apparatus.